The results of the previous section show that Galactic dust emission
in temperature can be subtracted to high accuracy from the
Planck spectra. However, dust subtraction leaves residuals at high
multipoles in temperature which arise from frequency dependent
extragalactic foregrounds. This necessarily requires that we fit for
frequency dependent extragalactic foregrounds described by `nuisance'
parameters. For polarization,
the situation is different since extragalactic foregrounds are
expected to be very weakly polarized. The high multipole polarization
measurements from ACTpol and SPTpol [1]}, [2]}
provide strong evidence that extragalactic infrared sources should be
undetectable in the Planck polarization power spectra (consistent
with what we see from the Planck data). In polarization, Galactic dust emission is
the only foreground that needs to be removed from the
Planck maps.
